In order to reduce the carbon dioxide emission and use of mineral oil and gas for the heating of buildings, thermal insulation is applied during the construction of new buildings and in the renovation of old buildings. Thermal insulation layers are in general placed internally, for example in a wooden roof construction. To avoid draft and also to protect the insulation material and the wooden construction against moisture normally at both sides of the insulation layer vapour controls may be placed, often in the form of membranes. Nevertheless moisture may penetrate into the roof construction, for example because of leakages through joints of the vapour control.
The vapour control placed at the outside of the roof construction may be in the form of a so-called roofing membrane or underlay. This vapour control ensures that no water in the form of rain, fog or snow penetrates the roof construction. This vapour control is highly permeable for water vapour to ensure that under all circumstances water that accumulates in the roof construction can evaporate from the roof construction.
It is important that the vapour control, which is arranged facing the inside of the building in winter time allows no or only a limited quantity of moisture to diffuse from the inside of the building into the insulation layers, where the moisture tends to condensate at the cold side of the insulation layers. During the summer however it is favourable if the vapour control, which is arranged facing the inside of the building is more permeable for water vapour to allow the insulation layers and the construction to dry from moisture by releasing the moisture also to the inside of the building.
For that reason in US-2004/0103604 a vapour control arranged at the inside of a building is proposed, which vapour control comprises a first layer having a water vapour diffusion resistance (Sd-value) of 2-5 meters diffusion-equivalent air space width, measured at a relative humidity of an atmosphere surrounding the layer of 30-50%, and having a Sd-value of <1 meters diffusion-equivalent air space width, measured at a relative humidity of 60-80%. In this way the vapour control has a high permeability for water vapour in summer, when ambient humidity is high and it has a low permeability for water vapour in winter time, when ambient humidity is normally low. A good example of a vapour control that fulfils these conditions is simply a polyamide film, since the diffusion constant of polyamide for water increases under humid conditions, due to the high water uptake of polyamide.
A problem however may occur where for example a kitchen or a bath room is present facing the vapour control. Because of the relatively high ambient humidity in such a room yet a high water transport takes place through the vapour control from the inside of the building, also in winter. This is of course especially true if the ventilation is poor and the use of the kitchen or bathroom is intensive. The water easily condenses in the isolation material and the roof construction and because of this fungi and rot may develop, causing bad smell and also damage of the roof construction.
Objective of the invention is to provide a vapour control that does not show this problem any more, while keeping enough capability to transport moisture to dry the insulation layers and the construction.
Surprisingly this objective is obtained by a vapour control arranged at the inside of a building is proposed, which vapour control comprises a first layer having a water vapour diffusion resistance (Sd-value) of 1-5 meters, preferably 2-5 meters diffusion-equivalent air space width, measured at a relative humidity of an atmosphere surrounding the layer of 30-50%, and having a Sd-value of <1 meters diffusion-equivalent air space width, measured at a relative humidity of 60-80%, comprising a second layer having a Sd-value of >0.2 meters diffusion-equivalent air space width, measured at a relative humidity of 80-100%, and the second layer is located at the side of the vapour barrier facing the inside of the building, relative to the position of the first layer.